This invention relates to drum brakes and is particularly although not exclusively concerned with internal drum brakes of the kind used with motor vehicles. It will be convenient to describe the invention with particular reference to internal drum brakes for use in motor vehicles, but it is to be understood that the invention has wider application.
The brake shoe used in internal drum brakes has traditionally been of a two-piece rigid type, or of a single piece band or loop type. The former type typically comprises a pair of substantially rigid shoes which are moved into engagement with the internal braking surface of a drum brake either by a plurality of actuators, or by a single actuator which acts at or toward one end of the brake shoes, with the opposite ends of the shoes being either in sliding or fixed pivoting abutment with a suitable abutment that projects from the backing plate. The latter type typically comprises a substantially cylindrical band having a gap defining opposed shoe ends, and which has a form promoting resilient flexibility. An actuator is operable at or adjacent the shoe ends to radially expand the band and cause engagement of friction lining disposed on a radially outer surface of the band against the internal braking surface of the drum and the resilient nature of the band acts as a spring to contract the band when the actuating force is released.
The above described brake shoes are separately fitted or mounted to assemblies that include a backing plate, against which the brake shoes are retained. Various means have been employed for this retaining function and in some forms of the two-piece shoe arrangement, the actuator and the abutment are fixed to the backing plate and their engagement with the respective brake shoe ends is such as to retain the brake shoes against the backing plate. In other arrangements, retaining clips have been employed to apply a retaining force to the brake shoe or pair of brake shoes and in still further arrangements, a combination of the above arrangements has been adopted.
Brake assemblies of the foregoing kind and particularly of the kind which employ two-piece brake shoes, are often of complex construction and are thus expensive and require careful manufacture and assembly. Moreover, such assemblies are prone to malfunction and wear because of corrosion or ingress of dust and dirt, due in part to the relative movement of the various parts.
It is an object of the present invention to provide either a brake shoe assembly for a drum brake which is of reduced complexity compared to known assemblies. It is a further object of the invention to provide an assembly in which the brake shoe is of a single piece construction, but which has similar operating characteristics to a rigid two-piece brake shoe construction. A still further object of the invention is to provide a new brake shoe assembly that can be actuated by known actuators. Yet a further object of the invention is to provide a method of making a brake shoe assembly for a drum brake.
According to the present invention there is provided a brake shoe assembly for a drum brake, said brake shoe assembly being of single-piece construction and including a brake shoe of generally circular form having two separate and opposed ends, and defining a pair of friction lining mounting sections disposed on opposite sides of said brake shoe which have a T-shaped cross-sectional form defining an arcuate, radially outer platform on which friction lining is mounted and a radially inwardly extending web, the cross-sectional form of each said lining mounting section causing said sections to have substantial rigidity in the plane of said web, said brake shoe including resilient hinge means between said lining mounting sections facilitating outward pivoting movement of said lining mounting sections to increase the separation between said brake ends between a brake operative condition in which in use, said friction lining engages a braking surface of drum to which the brake shoe assembly is fitted, and a brake inoperative condition in which there is no such engagement, a mounting member extending from said brake shoe in the region of said hinge means for connecting said shoe to a backing plate.
The above assembly is distinguished from prior art assemblies, in that the hinge means of the brake shoe assembly forms part of the single piece construction. Thus, the invention provides an assembly in which the lining mounting sections are of a substantially rigid construction as is normally only associated with a two-piece brake shoe assembly, but the assembly of the invention does not require a separate abutment to extend from the backing plate for the purpose of pivoting movement of brake shoes thereabout, as is required in the prior art. That is, in the invention, the shoes are formed as the lining mounting sections which are not separate components, but are part of a composite or integral construction which provides facility for the lining mounting sections to pivot relative to other parts of the assembly, and for which the torque reaction load under braking conditions is transmitted to the backing plate.
The lining mounting sections may be hinged by any suitable hinge means and that may include arranging them about a single hinge, or about a pair of hinges about which the lining mounting sections are separately or simultaneously pivotable. In a preferred form of the invention a pair of circumferentially spaced hinges, having reduced resistance to radial flexing of the brake shoe compared to the lining mounting sections are provided, and each lining mounting section is separately pivotable about a respective hinge.
The hinged arrangement of the lining mounting sections of the brake shoe is preferably one that provides for return movement of one or each lining mounting section, after an actuating force that shifts the section outwardly ceases to be operable. Thus, the hinged arrangement preferably has a resilience against such outward shifting, and that resilience may occur as a result of hinge construction, or may be a function of the material of which the hinged arrangement is made. Such an arrangement can reduce or eliminate the need for return springs or other equivalent systems, as are normally required in two-piece brake shoe assemblies.
The brake shoe assembly further includes means facilitating connection thereof to a backing plate and that means includes a mounting member to which the backing plate can be connected, and such connection can be by way of standard fasteners or by welding. The mounting member may be separately fixed to the assembly, but preferably it is integrally formed therewith. The mounting member preferably extends from the brake shoe assembly as a projection between the pair of circumferentially spaced hinges as provided in the preferred form of the brake shoe assembly.
In an alternative form of the invention, the brake shoe assembly is formed with a backing plate and that backing plate preferably depends from the mounting member which extends from the assembly, preferably between the pair of circumferentially spaced hinges as provided in the preferred form of the brake shoe assembly. The backing plate may be fixed to the mounting member by any suitable means as previously discussed, or alternatively it may be formed as an integral extension of that member.
The present invention further provides a method of forming a brake shoe assembly according to the invention, said method including deforming the edge of a planar circular disc (otherwise known as a shoe blank) to form an annular rim extending about the disc substantially perpendicular thereto and substantially centered relative to the portion of said disc inward of said rim, relatively severing and pressing sections of said disc to form said shoe ends, said lining mounting sections, said hinge means and said mounting member.
The present invention further provides a drum brake assembly that includes a drum rotatable about an axis and an inner cylindrical braking surface. Such a drum brake assembly includes a brake shoe assembly according to the invention and further includes actuating means being disposed between said lining mounting sections of said brake shoe assembly, which are operable to increase the separation between said brake shoe ends by pivoting movement of said lining mounting sections about said hinge means and expand said brake shoe from the brake inoperative condition to the brake operative braking condition (as hereinbefore defined) return means facilitating return movement of said lining mounting sections from said operative condition to said inoperative condition when said actuating means ceases to be operable to maintain said increased separation of said brake shoe ends, said backing plate extending from said mounting member for fixing to a drum brake assembly support.
A drum brake assembly according to the invention includes each of the advantages discussed earlier in relation to the brake shoe assembly, particularly the reduction in separate components, but the drum brake assembly of the invention is also considered to significantly reduce installation and maintenance times, and thus the costs associated with these procedures. It is envisaged that the brake shoe assembly of the invention will be fully replaceable on full friction lining wear, and this will provide an advantage over present known arrangements which require individual brake shoe removal and replacement, and thus the removal and replacement of the brake shoe assembly of the invention conveniently requires only unfastening of the whole worn brake shoe assembly, and replacement with a new assembly. The inconvenience of disassembling and reassembling a pair of brake shoes against a backing plate is therefore eliminated.